The present invention relates generally to a rotary atomizer for use with paint. More particularly, the invention pertains to an electrostatic rotary atomizer with indirect particle charging for use with water based conductive paints.
It is common practice to charge the paint droplets emitted from a rotary atomizer without directly charging the bell cup. These atomizers are often referred to as indirect or external charge systems and usually entail externally mounted electrodes. Externally charged systems have predominately been used for painting exterior surfaces of automobile bodies but are rarely used for painting the more intricate interior areas of the body. The limitation is primarily due to the size and geometry of the electrode ring.
Recent developments have proposed integrating the electrodes into the outside surface of the bell housing, however some shortcomings still exist. First, the electrodes can be easily contaminated because they attract paint particles and are placed in areas on the outside of the bell in proximity to the atomized cloud of paint particles. Second, the complex geometry of the electrodes does not facilitate easy cleaning and the electrodes are not compatible with automatic cleaning systems (smooth geometry best facilitates automatic cleaning). Third, externally mounted electrodes can be moved in the vicinity of a grounded part, or a portion of the automation which may become partially grounded. The electrode system is then susceptible to discharges, high current draw, or imbalanced charging.
For these reasons, paint application equipment suppliers have moved to direct charge systems, especially for interior waterborne bell painting systems. These direct charging systems generally require loading a volume of paint sufficient to paint the intended part and then isolating the stored volume from ground potential. These types of paint isolation systems are also called voltage blocking devices. Unfortunately, voltage blocking systems suffer from a longer color change time, higher color change waste and have a more complex fluid delivery system due to loading and isolation requirements.
Placing the electrodes inside of the bell housing resolves many of the shortcomings of the indirect external electrode charging and direct charging isolation systems. More specifically, fitting the electrodes in the shaping air supply chamber within the atomizer body will keep the electrodes clean and away from external ground points; furthermore the particle charging will be more uniform and balanced amongst the electrodes. When used in conjunction with water based (conductive) paints, relatively high transfer efficiency can be achieved without requiring external electrodes or a complex voltage blocking system. Many of the benefits of a simple fluid delivery system associated with direct charge solvent based systems can be incorporated into the internal inductive charge system.
An additional benefit can be claimed when using solvent-based paints in addition to water-based paints. In a particular implementation of the ionized air charging method, the charging ring can be insulated. Many of the metallic components are grounded minimizing the atomizer's capacitance. Having minimized capacitance and by hiding the electrodes, the high voltage is less likely to discharge to the grounded part. This particular charging implementation could offer improvements to automatic painting systems whether water based or solvent based paints are used.